Aside from genetics limiting the possible number of candidates, your forgetting it is isn't about mastery. It has been stated that Kekkei Genkai (elemental ones) are the simultaneous molding of two different chakra natures at the same time. It cannot be learned, it must be inherited. For there to ever be another Mokuton user, there'd be criteria. (1) Kishi would have to allow it. (2) They'd have to be of Senju lineage. (3) They would need both Doton and Suiton. (4) They'd have to be able use both elements simultaneously. (5) 3 does not necessarily mean 4.
The Senju clan also didn't have a specialty in Doton or Suiton. They were said to simply be a clan of a thousand skills. Hence why one was a world renown Genjustsu user and Tobirama a great Space-Time Ninjustu user.
Kimimaro was referred to by Orochimaru as the most powerful member of the Kaguya clan, having "abilities that even his war-loving clan feared". Because of this, he was locked away in a cage for most of his life, only released occasionally when needed to fight other clans.